1. Field of the Invention
The present invention relates to a product (including a machine, a manufacture, and a composition of matter) and a process (including a simple process and a production process). For example, in this specification, a semiconductor device, a method for driving the semiconductor device, a method for manufacturing the semiconductor device, and the like are described. In this specification, in particular, an imaging device including a photoelectric conversion element (e.g. an image sensor), a method for driving the imaging device, a camera including the imaging device, a surveillance system including the imaging device, and the like are described.
A semiconductor device in this specification refers to any device that can function by utilizing semi conductor characteristics. A semi conductor circuit, an imaging device, an electro-optical device, a display device, a light-emitting device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
An imaging device including a pixel array in which a plurality of pixels are arranged two-dimensionally has been known. A typical imaging device having such a configuration is a CMOS image sensor (also referred to as a MOS image sensor). A CMOS image sensor can be manufactured using a standard CMOS LSI process; therefore, an image sensor in which a controller, a circuit for reading an analog signal from a pixel array, a signal processing circuit for the analog signal, and the like are integrated into one chip together with the pixel array (the main body of the sensor) has been developed. The signal processing circuit for the analog signal is arranged to be compatible with a method for reading a signal of the pixel array.
For example, there is a method in which one row of a pixel array is selected and analog signals are read at the same time from pixels at the selected row through vertical signal lines of the respective columns. As such a CMOS image sensor which performs column-parallel processing, an image sensor provided with a column-parallel (column type) analog-digital conversion device has been proposed (for example, see Patent Document 1). In the image sensor disclosed in Patent Document 1, an analog-digital conversion device (hereinafter also referred to as ADC) is connected to an output of each vertical signal line.
Further, an image sensor has higher performance by having an image processing function. For example, in Non-Patent Document 1, an image sensor having a motion detection function is described. In Non-patent Document 1, a photo signal obtained through a first light exposure is stored in a pixel memory and then a second light exposure is performed. After a period for the second light exposure, a photo signal obtained through the second light exposure and a photo signal obtained by combining the photo signal which was obtained through the first light exposure and stored in the pixel memory and the photo signal obtained through the second light exposure are read. By difference calculation of the photo signals in a correlated double sampling (CDS) circuit, motion is detected.